Fix pk signature test framework bug.
Fix pk signature test framework bug.

Was attempting to test that the all-zero signature is invalid, then
tested some mutated signatures. Unfortunately it zeroed out the
signature before doing the mutation tests, so the mutated signatures
were all very-low Hamming weight strings instead of being close to
the original valid signature.

randombit/botan,randombit/botan,webmaster128/botan,randombit/botan,Rohde-Schwarz-Cybersecurity/botan,webmaster128/botan,webmaster128/botan,Rohde-Schwarz-Cybersecurity/botan,Rohde-Schwarz-Cybersecurity/botan,Rohde-Schwarz-Cybersecurity/botan,Rohde-Schwarz-Cybersecurity/botan,randombit/botan,randombit/botan,Rohde-Schwarz-Cybersecurity/botan,webmaster128/botan,webmaster128/botan